The Peace Corps said it is pulling more than 300 volunteers out of those countries temporarily.

The Peace Corps said the two volunteers had contact with a person who later died from Ebola. They have no symptoms and are currently isolated and under observation.

The Peace Corps said it is pulling more than 300 volunteers from Liberia, Sierra Leone and Guinea.

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The 340 volunteers are being removed from Liberia, Sierra Leone and Guinea out of an abundance of caution - 102 from Guinea, 108 from Liberia and 130 from Sierra Leone.

The outbreak has infected at least 1,200 people and killed nearly 700.

"Almost certainly those numbers are going to increase in the coming days and they probably represent an underestimate of the true number of cases because there's incomplete reporting of cases in most of the districts," said Dr. Stephan Monroe of the Centers for Disease Control and Prevention.

The virus is spread through direct contact and had an incubation period of up to 21 days. So a person could become infected, feel perfectly fine, travel elsewhere - then become sick and contagious.

The Ebola outbreak has infected at least 1,200 people and killed nearly 700.

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Theoretically, could the virus come over to the United States?

"Theoretically the virus could arrive in the United States and one of the things that we've done is to put out the health alert notice to just remind our health care providers in this country that if someone presents with symptoms that are compatible with Ebola virus, to ask them about recent travel history," Monroe said.

The CDC says even if the virus came to the U.S., it would be unlikely to spread here. But in West Africa, mistrust of doctors and lack of health care infrastructure are hampering efforts.

Do you reach a tipping point with an epidemic like this where there are so many people who are infected or are carrying the virus that it becomes really hard to control it?

"The outbreak is definitely at the point where it's hard to control," Monroe said. "It is true that this is going to be a long haul to bring this outbreak under control."